The response of Pseudomonas aeruginosa to phosphate starvation and subsequent refeeding was studied by following changes in turbidity, cell count and chemical composition on incubation in phosphate deficient medium. In shaken, phosphate deficient cultures, the turbidity and viable cell count were shown to increase significantly, with the latter reaching a maximum level at 24 hours under the conditions employed. A linear response of phosphate starved cells to low levels of phosphate supplied exogenously was evident from turbidity measurements, and a threshold requirement for phosphate analogous to the "energy of maintenance" (McGrew and Mallette, 1962) was not detected. In still, phosphate deficient cultures, the turbidity and total cell count increased and the viable cell count decreased slightly at 24 hours. The levels of protein and deoxyribonucleic acid (DNA) per ml of culture increased during this period, and the amount of ribonucleic acid (RNA) decreased. Extensive ribosomal degradation was apparent from sucrose density gradient centrifugation patterns. An enzyme having an alkaline pH optimum and displaying activity against a wide variety of phosphomonoesters was demonstrated in phosphate-starved cells. The enzyme was inhibited by inorganic phosphate, and was considered to be the counterpart of the repressible phosphomono-esterase reported in other microorganisms and studied in detail in Escherichia coli (Torriani, i960; Garen and Levinthal, I960; Heppel, Harkness and Hilmoe, 1962). The enzyme activity of cell free extracts of P. aeruginosa was shown to be associated mainly with the ribosomal fraction. / Land and Food Systems, Faculty of / Graduate

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